1. Field of the Invention
This invention is directed to lubricating oil compositions for use in railroad diesel engines having silver-surfaced engine parts and to methods for reducing wear of such silver-surfaced engine parts by use of such lubricating oil compositions.
2. State of the Art
Heavy duty diesel engines, such as railroad diesel engines, require crankcase lubricating oils which are stabilized against oxidation and which suspend combustion products. Combustion products lead to the formation of deposits in engines and the formation of sludge and varnish on pistons, cylinders, cylinder liners, and undercrown cavities. The diesel crankcase lubricant, in particular, should also prevent carbon deposition in the engine, especially in the top ring piston groove.
Railroad diesel engines and marine diesel engines are both heavy duty diesel engines. The internal components and operating conditions of these engines are, however, quite different. For example, two cycle railroad diesel engines are characterized by and distinguished from marine diesel engines by many factors including the presence of silver wrist-pin bearings which are operated under constant, extreme pressure lubricating conditions wherein these bearings operate to facilitate translation of the vertical movement of the pistons to circular movement of the crankshaft.
The use of a silver coating in the wrist-pin bearings is due to the fact that this metal imparts superior advantages in wear resistance to the bearing when operated under such extreme conditions. In contrast, marine engines of the cross-head type do not require silver coating of any of their internal parts.
As a further distinction, in railroad diesel engines, the lubricating oil composition is maintained in a sump and, during operation, this oil is constantly cycled to lubricate the internal parts of the engine. Contrarily, in marine diesel engines of the cross-head type, the lubricating oil composition employed to lubricate the piston ring-liner interface is kept separate from the sump and is employed as a "once through" oil.
In view of the differences in internal components and operating conditions existing between marine and railroad diesel engines, the lubricating oil composition requirements for these two types of diesel engines are also quite different. For example, marine diesel engines use an SAE 50 grade marine cylinder lubricant (MCLs), while railroad diesel engines use an SAE 40 grade lubricant. In addition, marine diesel engines burn fuel containing up to 4% sulfur (the OEM requirement) and thus require crankcase lubricating oils having a high alkalinity (on the order of 50-100 TBN) in order to neutralize acids formed during fuel combustion. By contrast, railroad diesel engines burn fuel having no more than about 0.5 weight percent sulfur and, therefore, require lubricating oils having a significantly lower alkalinity, i.e., a TBN generally of about 5 to 30, which is also employed to neutralize acids generated during fuel combustion.
In general, a TBN of from 5 to 30 for railroad lubricating oil compositions is achieved by inclusion of a requisite amount of a calcium overbased sulfurized alkylphenate. However, the inclusion of this highly overbased alkylphenate is detrimental to use in railroad diesel engines as it imparts undesirable wear to the silver coating on the silver wrist-pin bearings.
The art has recognized this problem and, heretofore, silver corrosion in railroad diesel engines was typically inhibited by the inclusion of a corrosion inhibiting amount of a chlorinated hydrocarbon. See, for example, U.S. Pat. Nos. 4,169,799, 4,171,269, 4,320,016, 4,428,850 and 4,464,276. However, the use of chlorinated hydrocarbons is perceived to be detrimental to the environment and causes disposal problems for spent oil. As a result, chlorinated hydrocarbons and their use are increasingly becoming subject to more stringent environmental regulations.
In view of the above, it would be highly desirable to discover a non-chlorinated lubricating oil additive which protects silver-surfaced diesel engine parts from wear and thereby permit the formulation of a lubricating oil composition suitable for railroad diesel engine use which composition is free of chlorinated hydrocarbons.
While the art has provided some examples of non-chlorinated additives which inhibit silver corrosion (e.g., U.S. Pat. Nos. 4,764,296, 4,734,211, and 4,820,431), additional additives which would inhibit silver wear in railroad diesel engines would be advantageous.
This invention is directed to the discovery that certain lubricating oil soluble hydrocarbyl polyoxyalkylene compounds are especially useful in inhibiting silver wear in railroad diesel engines. In this regard, it is noted that polyoxyalkylene compounds have been used as lubricating oil additives in compositions for use in the upper cylinder chamber of marine diesel cylinder engines of the crosshead type. For example, U.S. Pat. No. 4,402,845 discloses lubricant compositions having a TBN of from 50 to 100 which are used in marine diesel cylinder engines of the crosshead type and which contain a polyethylene glycol compound of the formula: EQU R--CH.sub.2 O--(CH.sub.2 CH.sub.2 O).sub.n H
wherein n ranges from 7 to 40 and R is an alkyl group containing from 11 to 15 carbon atoms. These compounds are disclosed as improving the spreadability of marine diesel engine cylinder oils after injection of the oil into the interior of the cylinder chamber. In column 2, line 40 of the patent, TERGITOL.RTM. 15-S-20 is cited as a particularly preferred compound.
Similarly, U.S. Pat. No. 4,479,882 discloses a process for improving the spreadability of a lubricating oil composition having a TBN of from 50 to 100 for use in a marine diesel engine of the crosshead type by incorporating therein of a polyalkoxylated phenoxy compound of the formula: ##STR1## wherein R is an aliphatic hydrocarbyl group having from 5 to 70 carbon atoms and n ranges from 14 to 30.
However, there is apparently no disclosure of using such polyoxyalkylene compounds or other hydrocarbyl polyoxyalkylene compounds of the type described herein below in railroad diesel engines for any purpose let alone for the purpose of inhibiting silver wear.